The present invention relates to superconducting shields, and more particularly to a high-transition-temperature superconductive multilayer shield, designed, fabricated, and intended for use in low magnetic field applications, e.g., superconducting electronics.
The invention also relates to the particular construction of such a superconducting shield and a novel method of making the same whereby precise structural configurations are obtained in the resulting shield and the effectiveness of the shield is greatly enhanced.
Superconductivity is a phenomenon occurring at very low temperatures in many electrical conductors, in which the electrons responsible for conduction undergo a collective transition to an ordered state with many unique and remarkable properties. These include the vanishing of resistance to the flow of electric current, the protection from penetration of magnetic flux and the expulsion of magnetic flux from the interior of a superconductive shielded device. The temperature at which the transition occurs is called the transition or critical temperature, T.sub.c.
There are two types of superconducting materials, appropriately named, "Type I" and "Type II". In Type I superconductors below a "critical magnetic field" H.sub.c which increases as the temperature decreases below T.sub.c, the magnetic flux is excluded. If the applied field is increased above H.sub.c, the entire superconductor reverts to the normal state and the field penetrates completely.
In Type II superconductors, there are two critical fields, the lower critical field H.sub.cl and the upper critical field, H.sub.c2. In applied fields less than H.sub.cl, the superconductor completely excludes the field, just as a type I superconductor does below H.sub.c. At fields just above H.sub.cl, however, flux begins to penetrate the superconductor in microscopic filaments called fluxoids or vortices. Each fluxoid consists of a normal core in which the magnetic field is large, surrounded by a superconducting region in which flows a vortex of persistent supercurrent which maintains the field in the core. In a sufficiently pure and defect-free type II superconductor, the fluxoids tend to arrange themselves in a regular lattice. This vortex state of the superconductor is known as the mixed state. It exists for applied fields between H.sub.cl, and H.sub.c2. At H.sub.c2, the superconductor becomes normal and the field penetrates completely.
A type II superconductor in the mixed state is not necessarily completely lossless, however. The presence of an electric current creates a force on the fluxoids. They therefore tend to move. Moving magnetic flux creates voltages by electromagnetic induction, and the presence of nonzero voltages together with the current implies power dissipation. This loss mechanism can often be suppressed by introducing defects into the crystal structure of the superconductor which tend to pin down the fluxoids and prevent them from moving. This phenomenon is called flux pinning.
Superconducting devices perform functions in the superconducting state that would be difficult or impossible at room temperature. One such device, the SQUID, Superconducting Quantum Interference Device, is used for detecting changes in magnetic flux. The SQUID employs superconducting magnetic shielding to isolate it from external magnetic fields. Without such shielding, changes in the external field can induce anomalous signals in the SQUID.
Superconducting shields for use in low magnetic field environments such as those surrounding field sensitive superconducting electronics, are generally fabricated from Pb or Nb. Of these two, Pb is usually the most reliable since it is the easiest to work with in the fabrication process and is easier to seal overlapping joints. It is also a clean, type I superconductor which exhibits good flux expulsion.
Nb shields are mechanically stronger and the Nb transition temperature is higher (9.2 Kelvin vs. 7.2 Kelvin). Hence, Nb shields are also frequently used. Some care must be taken however to ensure good overlapping joints at the ends of the shield. Also since Nb is a type II superconductor with stronger flux pinning forces than Pb the shields need to be carefully annealed to ensure minimal flux trapping.
Shields fabricated from NbTi are sometimes used since they are easier to machine than pure Nb. NbTi, however is an extreme type II superconductor with strong pinning forces making annealing even more important than in Nb. Also, being an alloy, composition gradients can exist in NbTi if it is not properly homogenized. Flux trapping and flux motion often are problems with NbTi shields if they are not carefully annealed to reduce the material inhomogeneities.
Shields fabricated from higher transition temperature superconducting compounds have been developed mainly for those applications requiring shielding of large magnetic fields (largely due to high values of trapped magnetic flux). The higher transition temperatures offer many advantages for low field applications as well and will become increasingly more important as superconducting electronics begin to operate at temperatures achieveable with small closed cycle refrigerators, i.e., 8-10 K. Even with operating temperatures of 4.2 K, higher T.sub.c shields are important in situations where precise temperature stability cannot be assured.
Achieving good shielding characteristics has proven difficult in prior art shields because of imperfect materials used in shield construction, difficulty fabricating magnetically "tight" shields with anything except lead (Pb), "trapped magnetic flux" during cool down, thermal drifts during temperature fluctuations, and random noise due to magnetic flux motion.